Setlite communication

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SATELLITE COMMUNICATIONS
LECTURE 5
Academic year 2021-2022
Eng. Murtile

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Satellite Link Design
Fundamentals

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In this chapter, as the title of the chapter itself suggests, a
comprehensive look will be taken at the important parameters
that govern the design of a satellite communication link. The
significance of each one of these parameters will be discussed
vis-`a-visthe overall link performance in terms of both quantity
and quality of services provided by the link, without of course
losing sight of the system complexity of both the Earth station
and the space segment and the associated costs involved
therein. What is implied by the previous sentence is that for a
given link performance, a better system performance could
perhaps be provided by making the Earth station and
spacecraft instrumentation more complex, thereby increasing
overall costs, sometimes to the extent of making it an unviable
proposition.

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The designer must therefore attempt to optimize the overall
link, giving due attention to each element of the link and the
factors associated with its performance. The chapter begins
with a brief introduction to various parameters that
characterize a satellite link or influence its design. Each one of
these parameters is then dealt with in greater detail. This is
followed by the basics of link design and the associated
mathematical treatment, which is suitably illustrated with a
large number of illustrations and design examples.

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Transmission Equation
The transmission equation relates the received power level at the
destination, which could be the Earth station or the satellite in
the case of a satellite communication link, to the transmitted RF
power, the operating frequency and the transmitter–receiver
distance. It is fundamental to the design of not only a satellite
communication link but also any radio communication link
because the quality of the information delivered to the
destination is governed by the level of the signal power received.
The reason for this is that it is the received carrier-to-noise ratio
that is going to decide the quality of information delivered, and
for a given noise contribution from various sources, both internal
and external to the system, the level of received power is vital

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Satellite Link Parameters
Important parameters that influence the design of a satellite
communication link include the following
1. Choice of operating frequency
2. Propagation considerations
3. Noise considerations
4. Interference-related problems

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Choice of Operating Frequency
The choice of frequency band from those allocated by the
International Telecommunications Union (ITU) for satellite
communication services such as the fixed satellite service
(FSS), the broadcast satellite service (BSS) and the mobile
satellite service (MSS) is mostly governed by factors like
propagation considerations, coexistence with other services,
interference-related issues, technology status, economic
considerations and so on. While it may be more economic to
use lower frequency bands, there would be interference-related
problems as a large number of terrestrial microwave links use
frequencies within these bands. Also, lower frequency bands
would offer lower bandwidths and hence a reduced
transmission capacity.

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Higher frequency bands offer higher bandwidths but suffer
from the disadvantage of severe rain-induced attenuation,
particularly above 10 GHz. Also, above 10 GHz, rain can
have the effect of reducing isolation between orthogonally
polarized signals in a frequency re-use system. It may be
mentioned here that for frequencies less than 10 GHz and
elevation angles greater than 5◦, atmospheric attenuation is
more or less insignificant.

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In both analogue and digital satellite communication systems,
the quality of signal received at the Earth station is strongly
dependent on the carrier-to-noise ratio of the satellite link.
The satellite link comprises an uplink, the satellite channel
and a downlink. The quality of the signal received on the
uplink therefore depends upon how strong the signal is, as it
leaves the originating Earth station and how the satellite
receives it.
Noise Considerations

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On the downlink, it depends upon how strongly the satellite
can retransmit the signal and then how the destination
Earth station receives it. Because of the large distances
involved, the signals received by the satellite over the
uplink and received by the Earth station over the downlink
are very weak.

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Interference-related Problems
Major sources of interference include interference between
satellite links and terrestrial microwave links sharing the same
operational frequency band, interference between two satellites
sharing the same frequency band, interference between two
Earth stations accessing different satellites operating in the
same frequency band.

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Interference between satellite links and terrestrial links
could further be of two types: first where terrestrial link
transmission interferes with reception at an Earth station
and the second where transmission from an Earth station
interferes with terrestrial link reception.

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Propagation Considerations
As outlined earlier, the nature of propagation of
electromagnetic waves through the atmosphere has a
significant bearing on the satellite link design. As will be seen
in the paragraphs to follow, it is the first few tens of kilometres
constituting the troposphere and then the ionosphere extending
from about 80 km to 1000 km that do the major damage.

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Attenuation is defined as the difference between the power that
would have been received under ideal conditions and the
actual power received at a given time.

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Free-space Loss
Free-space loss is the loss of signal strength only due to distance
from the transmitter. While free space is a theoretical concept of
space devoid of all matter, in the present context it implies
remoteness from all material objects or forms of matter that could
influence propagation of electromagnetic waves.
Propagation Considerations (cont..)

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Figure 1.1 Fading phenomenon

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The free-space path loss component can be computed
from

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If c is taken in km/s and f in MHz, then the free-space
path loss can also be computed from

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Attenuation due to Rain
Propagation Considerations (cont..)
After the free-space path loss, rain is the next major factor contributing
to loss of electromagnetic energy caused by absorption and scattering
of electromagnetic energy by rain drops.
Losses due to rain increases with an increase in frequency
and reduction in the elevation angle.

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Figure 1.2 Rain-caused attenuation as a function of frequency
and elevation angle

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where α = specific attenuation of rain in dB/km.
Specific attenuation again depends upon various
factors like rain drop size, drop size distribution,
operating wavelength and the refractive index.

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Cloud Attenuation
Propagation Considerations (cont..)
Attenuation due to clouds is more or less irrelevant for lower
frequency bands (L, S, C and Ku bands), but is largely relevant
for satellite systems employing Ka and V band frequencies.

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Example
Compute the free-space path loss in decibels for the
following conditions:
1. For a path length of 10 km at 4 GHz operating
frequency
2. Earth station transmitting antenna EIRP = 50 dBW,
satellite receiving antenna gain = 20 dB and received
power at satellite = −120 dBW

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(b) Path loss can be computed from: Received power = EIRP
+ receiving antenna gain – path loss Therefore, path loss =
EIRP + receiving antenna gain – received power = 50 + 20 −
(−120) = 50 + 20 + 120 = 190 dB

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THANKS
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